Energy enters a food web as sunlight captured by producers and flows one way along the arrows. At each step only about 10% is passed on; the rest is used for living or lost as heat. Matter is different: decomposers return it to be used again.
A field of grass can feed many grasshoppers, which feed fewer lizards, which feed a couple of kookaburras, which feed perhaps one eagle. Why the shrinking numbers?
Last lesson: the arrows. This lesson: what travels along them, and how much arrives.
Engage: Commit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Explore: Try it on the board and compare what happens with your prediction.
Explain: Use the particle model to explain what you saw.
Practise: A few on your own, to check you have it.
Elaborate: Use the same idea in a new context.
Evaluate: Look back at your first ideas and what changed.
WidthMore contexts, same idea.
Make your prediction first. Help opens once you've locked it in.
EngageCommit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Engage: how much gets through?
ExploreTry it on the board and compare what happens with your prediction.
Explore: follow the energy
Click a producer, then follow the arrows up, one organism at a time. Watch the energy under each organism.
Try it yourselfYour edits are saved in this browser.
Trace a food chain of at least four organisms, from a producer to the top.
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: flow, loss and cycling
In your book: explain why energy flows but matter cycles in an ecosystem.
A lizard gets 100 kJ from the grasshoppers it eats. About how much reaches a kookaburra that eats the lizard, in kJ?
Where does the other 90% go at each step?
Heat given off by the animal's body
Energy used for moving, breathing and keeping warm
Parts not eaten (bones, fur, roots)These go to decomposers, not up the chain.
Energy stored in the eaten animal's body
PractiseA few on your own, to check you have it.
Mix it up
Elaborate TypicalUse the same idea in a new context.
Elaborate: why so few eagles?
Elaborate WiderUse the same idea in a new context.
Elaborate: a desert and a rainforest
EvaluateLook back at your first ideas and what changed.
Evaluate: look back
Write one sentence: "At first I thought… Now I think… because…"
Revision summary
Energy enters as sunlight captured by producers and flows one way along the arrows.
At each step only about 10% is passed on; about 90% is used for living or lost as heat.
That's why there are few top predators.
Matter cycles: decomposers return atoms to the soil and air, and producers use them again. Energy does not cycle.
For teachers
Students trace energy through at least three trophic levels, explain that energy is lost at each level (about 90%), and distinguish one-way energy flow from matter cycling (AC9S7U02, node 2, a hinge node). About 25 minutes.
I can trace the direction of energy through at least three levels of a food web.
I can explain that energy is lost at each level.
I can explain the difference between energy flow and matter cycling.
Before class
A concept tutorial for introducing or revising the idea; it does not replace an energy pyramid built from real data. Set the width with ?width=minimum, typical (default) or wider.
The hinge risk is conflating energy flow with matter cycling: students then predict energy "coming back" in node 4.
Board notes: the board shows energy by feeding level with the 10% rule (10 000, 1 000, 100, 10 kJ). The 10% rule is an average; real transfers range from about 5% to 20%. Animals that feed at more than one level (the kookaburra) are shown at their highest level.
Running the lesson
Engage: most predict half or more. Write the chain on the board with each prediction.
Explore: ask students to trace two different chains and compare the energy at the top.
Explain: the sort names where the 90% goes; the third question separates energy from matter.
In your own words: the 'energy flows, matter cycles' paragraph is the evidence.
The hard part (on purpose)
Seeing that energy is lost at each step and not recycled, while matter is. When a student says decomposers recycle energy, ask what they give back to the soil.
When students get stuck
A student says decomposers recycle energy. Ask: What do decomposers give back to the soil: atoms or sunlight? Fix: Atoms (nutrients). The energy leaves as heat.
A student thinks the eaten animal passes on all its energy. Ask: What did the lizard use energy for while it was alive? Fix: Moving, breathing, keeping warm: that energy is gone before anything eats it.
A student reverses the arrows when tracing. Ask: Which has more energy, the grass or the lizard? Fix: The grass. Energy goes up the chain, getting smaller.
Stretch and support
Stretch: If humans ate grain directly instead of feeding it to cattle and eating the beef, how much more energy would reach us? Use the 10% rule.
Support: Give a four-row energy pyramid template to fill in while tracing.
Where it leads
Next: abiotic factors (node 3), then biotic factors and cascades (node 4), where energy flow explains why removing a species changes the others.
In class: build an energy pyramid with blocks (10 for grass, 1 for grasshoppers…); measure the heat from a compost heap.
What each step is for
Engage: how much gets through?: Students who think energy is passed on whole (or recycled) will choose those options; the trace makes the 90% loss visible.
Explore: follow the energy: Tracing a chain click by click, with the energy shrinking at each level, connects arrows to energy flow and loss.
Explain: flow, loss and cycling: Stating the one-way flow with losses, then contrasting it with matter cycling, stops the 'energy is recycled' error that breaks node 4 predictions.
Mix it up: Energy flows along the arrows between producers and consumers, so the roles in a web come back before counting the energy at each step.
Elaborate: why so few eagles?: Connecting energy loss to population size explains why top predators are rare, the link to node 4.
Elaborate: a desert and a rainforest: Comparing two ecosystems' energy input applies the flow principle to ecosystem structure.
Now you can
Trace the direction of energy through at least three levels of a food web.
Explain that energy is lost at each level.
Explain the difference between energy flow and matter cycling.